They probably failed one by one.
The usual cause is fatigue failure of the flexible braided connection between the speaker's fixed terminals at the back and the moving voice coil cone. It usually shows up initially by sounding "scratchy" and then eventually stops working.
In a 2001, aging of the cone material, especially at the outer circumference, can also cause failure of the speaker.
Some people rarely, if ever, use the radio and, when the car is sold, might not know that the speakers aren't working. Next owner then notices "no sound".
Here's another thread with the same problem that was the speakers: http://www.subaruoutback.org/forums/110-gen-2-2000-2004/49291-stereo-lack.html
